1. Technical Field
The invention relates generally to rotating cutting blades and methods for manufacturing rotary cutting blades. More particularly, the invention relates to cutting blades and methods for manufacturing cutting blades which provide cutting edges having a high hardness and which have necessary toughness to sustain high-energy impacts.
2. Related Prior Art
Rotary cutting blades are used in machinery such as lawn mowers, rotary cutting machines and agricultural equipment. Such rotary cutting blades include lawn mower blades, disc mower blades and straw chopper blades, each of which may have a cutting edge at one end of the blade. Such equipment normally includes one or more rotating blades driven at high speeds. Rotating cutting blades should have a sharp cutting edge at the leading edge of the outer periphery of the blade, and are required to withstand high speed impact with objects that may be encountered, including rocks, pieces of metal and other hard objects. More particularly, the rotating blades must be able to withstand such impact without shattering or otherwise fracturing.
Cutting blades are subject to wear by the fiber-like material being cut and are also subject to wear by sand and grit encountered by the blade. Increasing the hardness of the cutting blades improves resistance to wear due to normal cutting conditions. However, increasing the hardness of the cutting blade also tends to increase the brittleness of the blade and reduces the capacity of the blade to withstand impacts with hard objects. Further, cutting edges are subject to deformation by larger pebbles, rocks and other debris. This deformation has the effect of dulling the cutting edge without necessarily removing material from the blade. A dull cutting edge may be perceived as having been worn dull. Once the blade is perceived as dull, the user may sharpen and remove material from the blade, thereby reducing the life of the blade. Also, a dull cutting edge does not break the fiber off as cleanly and reduces the quality of cut.
Rotating blades used in lawn mowers are commonly required to comply with national standards, including standards promulgated by ANSI. In one such test, a blade is rotated at high speed in a test housing. Once the blade is rotating at high speed in a steady state, a metal shaft or post is quickly moved into the path of the rotating blade. The leading edge of the blade impacts the post and is stopped suddenly. In order to pass this test, the cutting blade must remain intact upon impact, i.e., pieces of the blades are not permitted to fly from the blade.
Another test that may be used in the production of rotary blades and which may indicate effectiveness of a cutting blade hardening process is a Charpy v-notch impact test according to ASTM standard E-23.
One way of manufacturing a rotary blade to meet the various design criteria is to heat treat the cutting blade after it is formed. It is commonly known to harden a rotary blade using a variety of methods. One such method is heat treating the entire body of the blade. Such heat treating processes, using an ordinary spring grade of carbon steel, can result in a blade having a hardness of 40-45Rc. Some heat treating processes can result in a blade having a hardness of 47Rc. However, cutting blades having a hardness of 47Rc or greater generally produce a blade that is too brittle to withstand impact.
Another known hardening process involves the spraying or depositing of tungsten carbide on the bottom surface of a rotary blade adjacent the leading or cutting edge. The application of such a coating to a blade can result in a surface hardness of approximately 70Rc. Such a spray-bonded coating is approximately 0.006 inches thick and results in a region of the blade being extremely hard with the remainder of the blade having a hardness of 40-45Rc after conventional heat treating.
A problem with spray-bonding a blade with tungsten carbide is that sand and grit abrade the spray-bonded surface. Fiber material such as grass being cut does not wear away the spray-bonded material, but sand and grit fractures the spray-bonded material and erodes the material away. Once the spray-bonded coating erodes, the remaining, unprotected cutting surface tends to quickly dull as it is not sufficiently hard. One method of addressing this problem is to pre-treat the blade with a matrix material to improve the bonding between the tungsten carbide and the steel blade. The matrix material has approximately a 50Rc and acts as an interface between the coating and the blade. However, this matrix material may also provide a weak area in the leading edge of the blade. Also, the sprayed on material and matrix can include voids and defects and is relatively brittle. Also, spray-bonding does not increase the life of a standard heat-treated rotary blade. Such spray-bonding of tungsten material is relatively expensive and can be difficult to obtain on an industrial level.